Vehicles having an internal combustion engine may be operated across a range of conditions over a drive cycle. As such, an engine may be configured to vary operation between one or more modes based on operating conditions to achieve various benefits of each mode of operation. For example, an engine may be configured to operate in a homogeneous charge compression ignition (HCCI) mode (i.e. a controlled autoignition or CAI mode) during some conditions in order to achieve improved fuel efficiency and/or reduced emissions as compared to a spark ignition (SI) mode. However, during some conditions, where it may be difficult to achieve reliable combustion in the HCCI mode, the SI mode may be selectively employed. In this manner, an engine may be configured to respond to operating conditions by selectively transitioning between SI mode and HCCI mode.
However, during some conditions, transitions between SI mode and HCCI mode may be difficult to achieve. For example, SI mode may utilize an initial charge of air and fuel at approximately ambient temperature, whereas HCCI mode may utilize an initial charge having a substantially higher temperature. The difference in operating conditions of these two modes may create undesirable discontinuities during transitions, such as torque disturbances, noise and vibration harshness (NVH), misfire, pre-ignition, and/or knock. Therefore, various control strategies have been aimed at improving the transition between these two modes of operation.
In one approach, as described in U.S. Pat. No. 6,336,436, combustion changeover between spark ignition combustion of a homogeneous mixture to compression autoignition combustion is achieved via an intermediate stratified lean spark ignition mode by varying the timings of intake and exhaust valves.
The inventors herein have recognized some disadvantages with this approach. Specifically, in the above approach, an intermediate stratified charge combustion mode is used to facilitate transitions between the homogeneous SI mode and the HCCI mode, which may result in a longer transition period, potential torque disturbances, and/or reduced efficiency, at least under some conditions. Further, the intake and exhaust valve timings are varied by mechanical valve actuation, which may limit how the intake and exhaust valve timings may be varied in response to the transition, thereby limiting other aspects of engine control, such as spark timing, for example.
A method of transitioning a cylinder of an engine from a spark ignition mode to a homogeneous charge compression ignition mode, comprising: before a transition, operating the cylinder with at least some overlap between an opening event of an intake valve and a closing event of an exhaust valve in the spark ignition mode; in response to a transition request, operating the cylinder with at least some negative overlap between an opening event of the intake valve and a closing event of the exhaust valve, retarding an opening timing and a closing timing of the intake valve, the opening timing retarded a different amount than the closing timing and retarding ignition timing; and further increasing negative overlap and performing the homogeneous charge compression ignition mode.
By utilizing such operation, transitions between homogeneous SI and HCCI modes may improved without requiring an intermediate stratified mode, although such an intermediate mode may be used, if desired.